Astronomer Misty Bentz would like you to know that black holes don’t suck. “They’re not cosmic vacuum cleaners going around and sucking everything in,” she says. “They just use gravity the same way everything else does.”
Instead of a cosmic drinking straw, a black hole is a place in the universe that is so massive and dense that anything caught in its significant gravitational pull is unable to escape.
Black holes have been in the news a lot lately, from the swarm found near the supermassive black hole at the center of our galaxy, to the fastest-growing black hole ever observed, which ingests the equivalent of the mass of our Sun every two days, to the most distant black hole ever detected, dating to the dawn of the universe. From their inception to their potential demise far in the future, black holes are a fascinating part of our universe. Here’s their story as we understand it now, from start to finish.
Birth:
A stellar-mass black hole starts its life with a death. It’s born when a star at least 10 times more massive than our Sun runs out of fuel, having already fused hydrogen into helium, and helium into other elements, from carbon and oxygen all the way up to iron deep in the star’s core. With a weighty metal heart it has nothing left to bind together. It’s reached the end of its lifespan, and it explodes, sending the outer layers out in a violent burst as the core collapses in on itself.
“If there’s enough mass there—three times the mass of the Sun in the center of the star—this will collapse into a black hole. We call those stellar mass black holes because they have a mass similar to a star,” says Bentz, an astronomer at Georgia State University.
This link between the birth of a black hole and the death of the star that formed it is a fairly common occurrence across the Universe. Stars and black holes are closely intertwined, especially in areas of the universe where star formation is churning along at high speed.
“It’s actually really common to find dead stars where the new stars are forming, because the most massive ones don’t live very long. They’re gone right away,” Bentz says. “The lifetime of a star depends on its mass. The most massive stars live much shorter lives because they just burn through their fuel very quickly.”
In what Bentz calls a ‘giant recycling program,’ the creation of black holes can actually spark the formation of new stars as well. When a group of new stars form, the most massive among them die out very quickly, exploding at the end of their short lives. “Those shockwaves compress more gas and dust to cause more stars to start forming. Then the most massive of those will live short lives and explode, which will send out new shockwaves and start forming more stars. It’s this chain reaction of the deaths of stars causing the births of brand new stars,” Bentz says.
But stellar-mass black holes are only a small part of the picture. Much weirder are supermassive black holes, giant beasts whose origins are far more obscure. They’ve been observed at the center of galaxies, including our own, and seem to have a slightly different way of forming than their smaller compatriots.
“A supermassive black hole as we see it now has a mass of a million or a billion times the mass of the Sun. But it didn’t start that way, it started smaller. So the question is, how did they form and how did they get that big?” Jillian Bellovary, a theoretical astrophysicist at Queensborough Community College says.
Astronomers know that supermassive black holes got really big, very fast, showing up around 13 billion years ago. At that point, Bellovary says, “we already see that there are black holes that have billion times the mass of the Sun. We know they existed really early in the universe, and that’s weird because there is all this mass in a very small space, and we want to know how it got there.”
“It’s a bit of a chicken or the egg kind of problem,” Bentz says. “In the very early universe it’s possible we formed black holes just from direct collapse of really over-dense regions. Maybe the material started collapsing gravitationally, and then kept collapsing all the way down into a black hole and didn’t actually form stars or anything.”
The other option is that maybe supermassive black holes got their start in early galaxies, as smaller black holes formed and coalesced in the center of infant galaxies.
The precursors to these early supermassive black holes were likely moderately sized to start with, says Bellovary, and would have had to be larger than a mere stellar mass black hole, which wouldn’t have been able to grow fast enough in such a short period of time to form the behemoths of the early universe that we’ve observed.
“The supermassive black hole has to get some sort of jump start, it can’t be too small when it forms because then it won’t have enough time to get huge. So it has to be medium-sized when it forms.” Bellovary says.
Researchers are still trying to figure out how those first black holes would have formed from the hot gas and dust of the early universe. Typically, when matter like that collapses together, it forms stars. So there may have been something different about the chemistry of the early universe that helped kindle those initial black holes.
“That gas in the early universe was probably only made of hydrogen and helium, because those were the only elements made in the Big Bang, and everything else was made inside of stars. If you don’t have stars yet, you can’t have any other elements yet,” Bellovary says. The chemistry of the early universe, as well as the motion, or lack of motion of the gas could have helped trigger black hole formation in those early eons.
Growing up
Black holes don’t just stay at the same size forever. They get their (undeserved) sucky reputation because things that fall into them can never get out, and are instead added to the collective mass of the black hole, letting it grow.
“It doesn’t matter if it’s gas that’s falling onto it, If it’s another star that gets ripped apart and falls onto it, if its a planet that got ripped apart and fell onto it—whatever goes in adds to the mass of the back hole. That accretion process, eating little bits of stuff over a long period of time, that’s one way that black holes grow over the history of the Universe.” Bentz says.
“The way we think they can grow most efficiently is by swallowing up gas or accreting gas,” Bellovary says. “Gas falls into a black hole like water drains down in a bathtub: it swirls around and goes into a drain. Gas acts similarly in a black hole. It’s gravitationally attracted to the black hole, but it’s moving, so it starts to thin into a disc around the black hole and eventually, it falls in.”
Acquiring gas might be the most efficient way to grow, but black holes don’t shy away from mergers. Collisions between black holes end with the two invisible masses uniting, something that scientists can observe with the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, which first detected gravitational waves from the merger of two black holes in 2015 (the announcement of the discovery came in 2016).
“For the first time we can learn things about the universe that don’t involve light. We’ve always been dependent on light and our eyes before this. Without light we wouldn’t know anything about the Universe. Light has been wonderful to us, but for the first time we can now see things that would be impossible to see with light, like merging black holes,” Bellovary says. “We would never know what happens without gravitational waves.”
There’s a huge size gap between supermassive and stellar mass black holes, where by all accounts there should be intermediate black holes—medium-sized, Goldilocks-approved black holes that fit just right in between their smaller and larger cousins.
The only problem is that researchers haven’t observed them yet. Source